1. Field of the Invention
The present invention relates to electrolyte membranes and fuel cells using the electrolyte membranes. More particularly, the invention relates to electrolyte membranes for fuel cells which can maintain a high voltage in a prolonged operation and/or an operation at high temperature and have excellent stability, and fuel cells using the electrolyte membranes.
2. Description of the Prior Art
As electric generating systems using clean hydrogen as the energy source, having high efficiency, causing no pollution and generating no warming gas such as CO2, fuel cells have recently received attention. Regarding such fuel cells, development research has been earnestly undertaken for the purpose of using the fuel cells as fixed equipment in homes or business places and as mobile equipment in automobiles.
Fuel cells are classified according to types of electrolyte membranes used therein, and they are divided into alkali electrolyte membrane type, solid polymer electrolyte membrane type, phosphoric acid type, molten carbonate type and solid electrolyte membrane type. In the solid polymer electrolyte membrane type and the phosphoric acid type, the charge exchange substance is a proton, so that the fuel cells of these types are also referred to as “proton exchange membrane fuel cells”.
Examples of fuels used for the fuel cells include hydrocarbon fuels, such as natural gas, LP gas, city gas, alcohol, gasoline, kerosine and gas oil.
The above hydrocarbon fuel is first converted into a hydrogen gas and a CO gas by a reaction such as steam reforming or partial oxidation, and the CO gas is removed to obtain a hydrogen gas. The hydrogen is fed to an anode and dissociates into protons (hydrogen ions) and electrons by the function of a metal catalyst of the anode. The electrons flow to a cathode through a circuit with doing work, while the protons (hydrogen ions) diffuse into an electrolyte membrane and flow to a cathode. At the cathode, water is produced from the electrons, the hydrogen ions and oxygen fed to the cathode, and the water diffuses into the electrolyte membrane. That is to say, the fuel cells utilize a mechanism to draw electric current during the production process of water from oxygen and hydrogen derived from a fuel gas.
Examples of the electrolyte membranes used for such fuel cells include a cation-exchange membrane which is based on polystyrene and has sulfonic acid group, a mixed membrane of fluorocarbon sulfonic acid and polyvinylidene fluoride, a membrane wherein trifluoroethylene is grafted on a fluorocarbon matrix, and a perfluorocarbon sulfonic acid membrane.
However, migration of protons through the electrolyte membrane comprising such an organic resin membrane, namely, ionic conductance of the membrane, depends upon a water content in the membrane. When a prolonged operation is carried out or a high-temperature operation at a temperature of about 80° C. or higher is carried out, the water content in the membrane is lowered, and as a result, ionic conductance is lowered to cause reduction of an output voltage.
On this account, Japanese Patent Laid-Open Publication No. 103983/1994 proposes a solid polymer electrolyte membrane fuel cell in which a compound having a phosphoric acid group is contained in a polymer membrane to allow the polymer membrane to exhibit excellent water retention properties and thereby which can be favorably employed at an operating temperature of 80° C. or higher.
In Japanese Patent Laid-Open Publication No. 143723/2001, an electrolyte membrane comprising an amorphous silica molded product containing phosphorus pentoxide is disclosed as an electrolyte membrane for a fuel cell favorably employable at an operating temperature of 80° C. or higher.
However, when the solid polymer electrolyte membranes thus proposed are used for a long period of time at a high temperature of 100° C. or higher, the water content in the membrane is lowered because of high temperature, or in case of an electrolyte membrane using a resin, proton conductivity is lowered by deterioration of the resin component to thereby reduce a voltage, resulting in a problem of lowering of cell performance.